1. Field of the Invention (Technical Field)
The present invention relates to an electrical circuit and method for creating flat-topped high power electrical pulses, particularly with the use of metal oxide varistors (MOVs).
2. Description of Related Art
Metal oxide varistors (MOVs) have been known and used for some time. The most typical use of MOVs is to protect electronic equipment from voltage spikes. The most common metal oxide varistors contain granules of zinc-oxide sandwiched between a pair of conductive plates. Typically, one lead of a varistor is connected to a supply line (Vs) and another lead of the MOV is connected to ground. Under typical operating conditions, only a very small amount of current flows from one conductive plate, through the zinc-oxide, and to the other conductive plate. This is because of the relatively high resistance of the zinc-oxide. However, when a voltage spike occurs at a voltage which is greater than the breakdown voltage of spaces around the zinc-oxide granules, current then easily flows around the zinc-oxide granules and the MOV thus acts as a short from Vs to ground. The voltage at which this occurs is known as the clamping voltage of the MOV. When the MOV acts as a short to ground, the vast majority of all current that is supplied to the electronic device stops flowing to the device and instead travels through the MOV to ground. Because most voltage spikes that an electronic circuit is likely to experience typically last for only a small fraction of a second, the amount of time that the MOV acts as a short to ground is correspondingly very short and thus the MOV does not typically overheat.
One of the most demanding problems for systems designed to generate high power microwave and charged particle beams is the production of the required electrical pulses. Typical voltages are in excess of several hundred thousand volts at currents of about 10,000 amps or more for pulses lasting up to about 1 uS in duration. These pulses must maintain voltage within fairly narrow constraints (typically +/−10 percent) in order to be useful. In addition, the time for the voltage to reach a maximum after application (the rise time) must be rather short (often less than about 100 nanoseconds). A number of techniques are typically employed to accomplish this task. These include pulse forming networks and pulse forming lines using liquid or solid dielectrics.
Recently, MOVs have begun to be used in pulsed power applications (see Simulation, Design and Test of a MOV Pulse Shaping Device for High Power Microwave Generators, by M. Giesselmann, Pulsed Power Conference, 1999. Digest of Technical Papers. 12th IEEE International, Volume: 2, PP. 27-30 Jun. 1999; and Voltage Regulation in a Linear Induction Accelerator Using Metal-Oxide Varistors, by W. M. Parsons, IEEE Conference Record of the 1990 Nineteenth Power Modulator Symposium PP. 544-547). These references fail to describe the ability to repeatedly shape numerous pulses with a MOV, particularly one which is driven by a Marx-type generator. In addition, the prior art fails to adequately address the need for a peak smoothing system which also provides for a very rapid rise time.
There currently exists a need for an inexpensive manner by which smooth-topped, high-power pulses can be produced and particularly for a method to produce flat-topped, high-power voltage pulses having a very rapid rise time.